The preferred embodiment of this invention is related to motion control of an object, such as a shuttle carriage of an ink jet printer. However, it is generally applicable to multiphase motors which drive a mass.
In order to obtain accuracy, such as during a printing operation when typically 300 dots per inch are applied to a paper on which an image is being printed, velocity of the printer carriage must be closely controlled. Typically, a desired velocity profile has acceleration and deceleration ramps at the ends of a target, and a generally constant velocity. This velocity profile is fed directly to the motor during the scan over the paper. However, the motor and the carriage-drive apparatus are not absolutely rigid. A carriage is typically driven with a cable and pulley system or a cogged timing belt. Since the timing belt or cable have limited stiffness, the actual velocity profile of the carriage is not perfectly constant. Rather, it has undesirable ripple which causes print defects.
The cable or belt is thus analogous to a spring with a dashpot and the carriage is the mass. However, as it turns out in the spring analogy, the spring and dashpot are not anchored to a fixed base. Instead, they are attached to the rotor of the motor which is driven by the motor drive current. The result is that the spring, which is representative of the cable or belt, is attached to a mass represented by the rotor. The rotor is then fixed to a base through an electromagnetic connection between the motor stator and rotor, as a function of the motor current. The electromagnetic connection may also be represented by a second spring. Most of the ripple in the carriage is due to variations between the actual motor movement and the ideal or input motor movement defined by the motor current.
In a printer, a servo motor is often used to translate the carriage back and forth across a frame. Typically, a speed servo loop is formed around the motor with an encoder or tachometer used to provide a speed signal. A disadvantage of this approach is that the mass/spring combination formed by the heavy carriage and the elastic cable or belt are outside the loop, so the actual point of use is uncontrolled.
It is therefore desirable to place the control loop around the carriage so that the actual point of use is controlled. However, this is not easily done, since the mass/spring resonant system formed by the cable or belt and carriage make loop compensation very difficult.